Researchers have recently shed light on how exercise benefits the body on a cellular level. What’s more, they’ve determined a type of exercise that’s best for boosting cell health.

Published in Cell Metabolism (2017; 25 [3], 581–92), the study included 36 men and 36 women categorized as “young” (aged 18–30) or “older” (aged 65–80). Each participant was assigned to one of three training programs for 12 weeks:

high-intensity interval training (HIIT) on an indoor bike;

strength training with weights;

or a combination of the two.

Study leaders took muscle biopsies from the volunteers—who also underwent lean-mass and insulin-sensitivity tests—and then compared the results with those from a sedentary control group.

Data showed that the exercise groups experienced improvements in cellular function and in the ability of mitochondria to generate energy; this adds further evidence that exercise does in fact slow the aging process at a cellular level. Muscle mass and insulin sensitivity improved with all three training protocols. However, outcomes did vary.

“HIIT revealed a more robust increase in gene transcripts than other exercise modalities, particularly in older adults,” the authors explained. Specifically, HIIT increased mitochondrial capacity by 49% in the “young” group and 69% in the “older” group.

The authors added, “HIIT reversed many age-related differences in the proteome, particularly of mitochondrialproteins in concert with increased mitochondrial protein synthesis.”

For best benefit, according to the study, a combination of strength training and HIIT is recommended. While HIIT proved best at improving cellular health, it was less effective at increasing strength and muscle mass than the strength training protocol.

“We encourage everyone to exercise regularly, but the take-home message for aging adults is that supervised high-intensity training is probably best, because, both metabolically and at the molecular level, it confers the most benefits,” concluded K. Sreekumaran Nair, MD, PhD, a Mayo Clinic endocrinologist and senior researcher on the study.

Sure, you could eat whatever you wanted in high school and stay thin as a rail. But unfortunately you’re not 17 anymore, and even if you work out ‘round the clock, you can’t transform your body if you constantly give in to cravings, high-fat foods, and sweet treats. The truth is, flat abs are made in the kitchen and no amount of cardio and crunches can sculpt a sleek physique if you maintain an unhealthy diet. “Consume excess calories and you have to counterbalance them,” says Sara Haas, R.D.N., spokesperson for the Academy of Nutrition and Dietetics. “And it’s hard to get enough exercise in to undo the calories you’ll get in a double cheeseburger with French fries and a milk shake.”

Why You Can’t Out-Train a Bad Diet

You’re not a professional athlete If you justify frequent fried chicken or pasta dinners with the Flywheel class or CrossFit WOD on your schedule the following morning, you might want to recalculate how many calories you’re actually burning in comparison to the ones you’re taking in. “The majority of people are not serious athletes, meaning they don’t require the same type and amount of fuel as the pros,” says Haas. “Eating a calorically dense, high-carbohydrate meal or snack makes sense for a competitive cyclist about to endure a 100-mile road race, but it doesn’t make sense for someone who is about to take a two-mile jog around the block.” Instead, opt for a healthier form of chicken such as grilled or poached and save the spaghetti for a post-race meal. Yes, it’s OK to have a cheat here and there, but try not to make it a weekly or even bi-weekly thing.

And don’t bother justifying it with an intense sweat session you may have had earlier—it’s called a cheat meal for a reason. You won’t be able to hit your peak if you’re overdoing it with the wrong foods

To effectively change your physique and stay toned requires intense exercise. You won’t have the physical endurance to push through tough workouts if your diet isn’t up to snuff. Yes, that unfortunately means that while Reese’s Pieces and soda may give you a sugar high that you mistake as energy, they won’t fuel you to PR on the bench or around the track. Also, if you’re consuming high-fat foods in the evening, they could be disrupting your sleep, according to Brazilian researchers—which will leave you too tired to go all out at the gym.

You’ll need a combination of carbohydrates and protein to recover following a workout, as well as adequate carbs beforehand, too. “They’re the preferred energy for the exercisers’ muscles and mind,” says Jennifer McDaniel, R.D.N., founder of McDaniel Nutrition Therapy in St. Louis and spokesperson for the Academy of Nutrition and Dietetics.

Watch out for taking in too much fat; that often translates to an abundance of calories as well, which quickly packs on as extra pounds. Another diet pitfall to avoid when training is extremely high amounts of carbs or fiber. “These could cause annoying digestive issues and prevent you from performing well,” says McDaniel. In general, aim to get about 30 percent of your diet from protein, 40 percent from carbs and 30 percent from fat.

You won’t have the energy to exercise if you’re not eating enough

Keep in mind that a diet you might think is healthy—one that’s super-low in carbs or calories—is just as harmful to your workout plan as one that’s high in fat. A very restrictive eating plan, paired with hardcore exercise, could leave you leaning on muscle mass for energy, says McDaniel. Not getting enough fat (fat-soluble vitamins, like vitamins A and D, and essential fatty acids, like omega-3s, in particular), she adds, “leaves you unable to produce energy and grow muscle because it lowers your levels of hormones like insulin and testosterone—which are important for building lean-body mass.”

Additionally, maintaining a very restrictive diet for a prolonged period can lead to a reduction in muscle tissue and can decrease the ability of your skeletal muscles (the ones needed for lifting, walking, and other forms of exercise) to perform well, found a study published in the journal Advances in Nutrition.

You won’t want to exercise

Unhealthy food choices—whether you’re eating too much fat, too many calories, or not enough of either—may make you feel slow and less driven to exercise. “Diet and exercise are a feedback loop,” says McDaniel. “When you eat well, you are motivated to move, and when you move, you are more motivated to eat better.” Consider, for instance, a low-carb, high-fat diet; it might not only weaken training adaptations and hinder performance, McDaniel adds, but can also lead to a ‘hangry’ mood. Translation: You’ll be less likely to want to get to the gym.

You won’t be able to tone your target areas

Having a hard time sculpting a better butt or washboard abs? When you consume excess calories and can’t burn them all off solely from your workouts, they head right to these trouble zones. “It’s dependent on your specific body type,” says Haas, “but generally, women tend to gain weight in the hips and thighs, while men pack it on around their midsection.” So even if you’ve gained muscle in these areas, it will be covered by a layer of fat. And abs exercises alone aren’t enough to decrease your body-fat percentage or abdominal fat, according to a study published in the Journal of Strength and Conditioning Research.

Keeping your diet in check will help because to become truly toned, you’ll need to build muscle and burn more calories than you’re consuming at the same time. It’s easier to do so if you don’t treat yourself to nachos or ice cream in the first place.

You could get sick—or hurt

Low-carb and low-fat diets can be mentally draining and have a negative impact on heart health, says McDaniel. Plus, she says, “Following a chronic low-carb diet may lead to micronutrient deficiencies and increased inflammation throughout the body, which both make you more susceptible to injury.” Studies have found that not taking in an adequate amount of healthy fats may raise your chances for overuse injuries (such as stress fractures and tendonitis) and it doesn’t allow your body to protect itself in order to stay healthy. Furthermore, if you pair a low-fat diet with intense exercise, that can lower your immunity even further.

A diet of fiber-rich foods, such as fruits and vegetables, reduces the risk of developing diabetes, heart disease and arthritis. Indeed, the evidence for fiber’s benefits extends beyond any particular ailment: Eating more fiber seems to lower people’s mortality rate, whatever the cause.

That’s why experts are always saying how good dietary fiber is for us. But while the benefits are clear, it’s not so clear why fiber is so great. “It’s an easy question to ask and a hard one to really answer,” said Fredrik Bäckhed, a biologist at the University of Gothenburg in Sweden.

He and other scientists are running experiments that are yielding some important new clues about fiber’s role in human health. Their research indicates that fiber doesn’t deliver many of its benefits directly to our bodies.

Instead, the fiber we eat feeds billions of bacteria in our guts. Keeping them happy means our intestines and immune systems remain in good working order.

In order to digest food, we need to bathe it in enzymes that break down its molecules. Those molecular fragments then pass through the gut wall and are absorbed in our intestines.

But our bodies make a limited range of enzymes, so that we cannot break down many of the tough compounds in plants. The term “dietary fiber” refers to those indigestible molecules.

But they are indigestible only to us. The gut is coated with a layer of mucus, atop which sits a carpet of hundreds of species of bacteria, part of the human microbiome. Some of these microbes carry the enzymes needed to break down various kinds of dietary fiber.

The ability of these bacteria to survive on fiber we can’t digest ourselves has led many experts to wonder if the microbes are somehow involved in the benefits of the fruits-and-vegetables diet. Two detailed studies published recently in the journal Cell Host and Microbe provide compelling evidence that the answer is yes.

In one experiment, Andrew T. Gewirtz of Georgia State University and his colleagues put mice on a low-fiber, high-fat diet. By examining fragments of bacterial DNA in the animals’ feces, the scientists were able to estimate the size of the gut bacterial population in each mouse.

On a low-fiber diet, they found, the population crashed, shrinking tenfold.

Dr. Bäckhed and his colleagues carried out a similar experiment, surveying the microbiome in mice as they were switched from fiber-rich food to a low-fiber diet. “It’s basically what you’d get at McDonald’s,” said Dr. Bäckhed said. “A lot of lard, a lot of sugar, and twenty percent protein.”

The scientists focused on the diversity of species that make up the mouse’s gut microbiome. Shifting the animals to a low-fiber diet had a dramatic effect, they found: Many common species became rare, and rare species became common.

Along with changes to the microbiome, both teams also observed rapid changes to the mice themselves. Their intestines got smaller, and its mucus layer thinner. As a result, bacteria wound up much closer to the intestinal wall, and that encroachment triggered an immune reaction.

After a few days on the low-fiber diet, mouse intestines developed chronic inflammation. After a few weeks, Dr. Gewirtz’s team observed that the mice began to change in other ways, putting on fat, for example, and developing higher blood sugar levels.

Dr. Bäckhed and his colleagues also fed another group of rodents the high-fat menu, along with a modest dose of a type of fiber called inulin. The mucus layer in their guts was healthier than in mice that didn’t get fiber, the scientists found, and intestinal bacteria were kept at a safer distance from their intestinal wall.

Dr. Gewirtz and his colleagues gave inulin to their mice as well, but at a much higher dose. The improvements were even more dramatic: Despite a high-fat diet, the mice had healthy populations of bacteria in their guts, their intestines were closer to normal, and they put on less weight.

Dr. Bäckhed and his colleagues ran one more interesting experiment: They spiked water given to mice on a high-fat diet with a species of fiber-feeding bacteria. The addition changed the mice for the better: Even on a high-fat diet, they produced more mucus in their guts, creating a healthy barrier to keep bacteria from the intestinal walls.

One way that fiber benefits health is by giving us, indirectly, another source of food, Dr. Gewirtz said. Once bacteria are done harvesting the energy in dietary fiber, they cast off the fragments as waste. That waste — in the form of short-chain fatty acids — is absorbed by intestinal cells, which use it as fuel.

But the gut’s microbes do more than just make energy. They also send messages.

Intestinal cells rely on chemical signals from the bacteria to work properly, Dr. Gewirtz said. The cells respond to the signals by multiplying and making a healthy supply of mucus. They also release bacteria-killing molecules.